my ex employer still has me on their payroll since October 2023

My ex employer still have me on their payroll since October 2023, so HMRC thinks I have 2 jobs, whilst I only have 1. I have been under the TAX CODE of 111T M1 and because of this situation I have been overpaying TAXES for months. I have my P45 and it says the correct date I left my ex job, so I dont understadn what happened here, is this because they never informed HMRC about me leaving the job and sent to HMRC my P45??? Please, how can I fix this ASAP as since October I have lost money because of this mistake? is my ex employer who has to do something about this or my actual employer?

Hi,
You will need to contact HMRC to review/update your tax code.
If overpaid an amended code will be issued to your current employer which will then generate the refund due. 
Thankyou.